Telomeres and replicative cellular aging of the human placenta and chorioamniotic membranes

Recent hypotheses propose that the human placenta and chorioamniotic membranes (CAMs) experience telomere length (TL)-mediated senescence. These hypotheses are based on mean TL (mTL) measurements, but replicative senescence is triggered by short and dysfunctional telomeres, not mTL. We measured short telomeres by a vanguard method, the Telomere shortest length assay, and telomere-dysfunction-induced DNA damage foci (TIF) in placentas and CAMs between 18-week gestation and at full-term. Both the placenta and CAMs showed a buildup of short telomeres and TIFs, but not shortening of mTL from 18-weeks to full-term. In the placenta, TIFs correlated with short telomeres but not mTL. CAMs of preterm birth pregnancies with intra-amniotic infection showed shorter mTL and increased proportions of short telomeres. We conclude that the placenta and probably the CAMs undergo TL-mediated replicative aging. Further research is warranted whether TL-mediated replicative aging plays a role in all preterm births

Expression of NES-hTERT in Cancer Cells Delays Cell Cycle Progression and Increases Sensitivity to Genotoxic Stress

Telomerase is a reverse transcriptase associated with cellular immortality through telomere maintenance. This enzyme is activated in 90% of human cancers, and inhibitors of telomerase are currently in clinical trials to counteract tumor growth. Many aspects of telomerase biology have been investigated for therapy, particularly inhibition of the enzyme, but little was done regarding its subcellular shuttling. We have recently shown that mutations in the nuclear export signal of hTERT, the catalytic component of telomerase, led to a mutant (NES-hTERT) that failed to immortalize cells despite nuclear localization and catalytic activity. Expression of NES-hTERT in primary fibroblast resulted in telomere-based premature senescence and mitochondrial dysfunction. Here we show that expression of NES-hTERT in LNCaP, SQ20B and HeLa cells rapidly and significantly decreases their proliferation rate and ability to form colonies in soft agar while not interfering with endogenous telomerase activity. The cancer cells showed increased DNA damage at telomeric and extra-telomeric sites, and became sensitive to ionizing radiation and hydrogen peroxide exposures. Our data show that expression of NES-hTERT efficiently counteracts cancer cell growth in vitro in at least two different ways, and suggest manipulation with the NES of hTERT or its subcellular shuttling as a new strategy for cancer treatment

Senescence is an endogenous trigger for microRNA-directed transcriptional gene silencing in human cells

International audienceCellular senescence is a tumour-suppressor mechanism that is triggered by cancer-initiating or promoting events in mammalian cells. The molecular underpinnings for this stable arrest involve transcriptional repression of proliferation-promoting genes regulated by the retinoblastoma (RB1)/E2F repressor complex. Here, we demonstrate that AGO2, RB1 and microRNAs (miRNAs), as exemplified here by let-7, physically and functionally interact to repress RB1/E2F-target genes in senescence, a process that we call senescence-associated transcriptional gene silencing (SA-TGS). Herein, AGO2 acts as the effector protein for let-7-directed implementation of silent-state chromatin modifications at target promoters, and inhibition of the let-7/AGO2 effector complex perturbs the timely execution of senescence. Thus, we identify cellular senescence as the an endogenous signal of miRNA/AGO2-mediated TGS in human cells. Our results suggest that miRNA/AGO2-mediated SA-TGS may contribute to tumour suppression by stably repressing proliferation-promoting genes in premalignant cancer cells