Cellular senescence, cell death Jaromir Gumulec, Dept. of Physiology, Masaryk University Cellular senescence • What is it? • What causes it? • Why is it important (cancer and aging)? 0.000% 0.001% 0.010% 0.100% 1.000% 10.000% 100.000% 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030 relative publication number per field senescence_% cancer_% miRNA% polymorphism% deep learning_% apoptosis_% autophagy_% microbiome_% immunotherapy_% CRISPR_% phage display_% NGS_% 0.000% 0.001% 0.010% 0.100% 1.000% 1988 1993 1998 2003 2008 2013 2018 relative publication number per field (crop) senescence_% miRNA% deep learning_% autophagy_% microbiome_% CRISPR_% phage display_% NGS_% Cellular senescence What it is? • proces causing arrest of proliferation of cells in response to stressors • Important proces for physiology and pathology • Contributor to aging and age-related deseases • attractive therapeutic target G0 arrest ≠ senescence Senescence vs. quiescence vs. terminal differentiation reversible G0 arrest = (quiescent) stem cells reside in the quiescent state and enter the cell cycle upon activation irreversible „G0 arrest“ = terminally differentiated cells G1, S, G2 irreversible arrest = (senescent) Senescent cells are dysfunctional cells that have ceased proliferation and are permanently withdrawn from the cell cycle SENESCENCE mechanism of regulation of damaged cells mechanism in physiology (embryogenesis) Causes of cellular senescence • Caused by multiple factors Collado et al. Nature Reviews Cancer 6, 472–476 (June 2006) | doi:10.1038/nrc1884 Mechanisms of senescence: What causes it? • Telomere shorteing (replicative senescence) • CDKN2A locus derepression • Stress-induced senescence • Oncogene-induced senescence • Senescence-associated secretory phenotype p53 „decision making“ • supressor regulating DNA damage protection and uncontrolled growth, and others • DNA damage triggers p53 stabilization and activation • not completely clear what direct to apoptosis vs senescence • Fibroblasts and epithelial cells > senescence • lymphocytes > apoptosis • p53 acetylation-related mechanisms Telomere • A telomere is a region of repetitive nucleotide sequences at each end of a chromosome. • It protects the endof chromosome from deterioration of fusion with neighbouring chromosomes Telomere shortens as cells divide In most somatic tissues, telomerase is expressed at very low levels or not at all -- as cells divide, telomeres shorten Telomerase and Senescence Short telomeres signal cells to senesce (stop dividing) Telomerase and Cancer The presence of telomerase in cancer cells allows them to maintain telomere length while they proliferate Senescenceassociated secretory phenotype • Complex pro-inflammatory response • Senescent cells produce various factors, including IL- 6, NF-kB, TGF-B, GM-CSF, etc.) • This causes inflammation with subsequent phagocytosis of senescent cells CDKN2A locus derepression • = tumor supressor • Normally expressed at very low levels • Derepressed with ageing (indicator of biological age) Stress-induced sensescence • Reactive oxygen species increase after various stresses, incl. Chemotherapy • Antioxidant treatment prevents senescence DNA-damage response (DDR): cell fate following senescence triggering How to identify senescence Cellular senence vs. aging • Aging = progressive loss of organ function over time • organismal fitness declines • Antagonistic pleiotropy: Some traits selected to optimize fitness in young organisms can have unselected deleterious effects in old organisms (what's good for you when you're young may be bad for you when you're old) • genes related to aging were not evolutionary selected • „wear and tear“ theory: mainatenance of body integrity is extremely dificult to perfom, can only be performed at the expense of growth and reproduction. Species with a high risk of pedators rather vote reproduction strategy 100% Survivors Natural environment: predators, infections, external hazards, etcMost of human evolution Modern, protected environment (very VERY recent) Age Antagonistic pleiotropy Cellular senescence Selected for tumor suppression (growth arrest) Functional changes unselected, deleterious FUNCTIONAL CHANGES ASSOCIATED WITH CELLULAR SENESCENCE: Secretion of molecules that can be detrimental to tissues if not controlled e.g., senescent fibroblasts secrete proteases, growth factors, inflammatory cytokines Cellular senescence and aging •Cells from old donors divide less often than cells from young donors •Cells from short-lived species are more sensitive to senescence-inducers, particularly oxidative stress, than cells from long-lived species •Cells from donors with premature aging syndromes senesce more readily than cells from normal donors •Senescent cells (expressing a senescence marker) accumulate with age and at sites of age-related pathology EPITHELIUM Basement Membrane STROMA Senescent Epithelial Cell Senescent Fibroblast EPITHELIUM Basement Membrane STROMA YOUNG TISSUE OLD TISSUE Degradative enzymes, Inflammatory cytokines, etc. AGING ? Senescence vs. apoptosis Abl1 Abl1 Akt3 Arg2 Arntl Arntl Bcl2l12 Bcl6 Bmpr1a Calr Calr Cav1 Cdk6 Cdkn1a Cdkn2a Cdkn2a Cdkn2a Cdkn2a Cdkn2b Cgas Eef1e1 Fbxo5 H2-M3 Hmga2 Hras Icmt Id2 Ing2 Kat6a Kras Map2k1 Map3k3 Mapk14 Mapkapk5 Nampt Nek4 Nsmce2 Nuak1 Opa1 Pawr Pla2r1 Plk2 Pml Pnpt1 Prkcd Prkdc Prmt6 Rbl1 Rsl1d1 Sirt1 Sirt1 Sirt1 Slc30a10 Smc5 Smc6 Srf Suv39h1 Tbx2 Tbx3 Terf2 Tert Trp53 Trp53 Trp63 Twist1 Vash1 Wnt16 Ypel3 Zfp277 Zkscan3 Zmpste24 Zmpste24 Senescence in physiological processes • Role in Embryonic development and normal adult cells • Embryo: mesonephros involution, • Importance of apoptosis during embyo well defined • Adult: human placenta = large syncytium - endoreruplicaition Polyploidy • during mitosis, cells with diploid chromosome sets are created (46XY) • polyploidy = number of chromosome sets > 2 • physiological in plants (higher gene copy numbers) • role in pathology in humans (consequence of defects during cell division) • organism aging • oncogenesis • trigering DNA-Damage response pathway • physiologically in humans in specialzed cells only • megakaryocytes • cardiomyocytes (during hypertrophy) • skeletal muscle cells • during stress response (oxidative stress, toxins exposition) • better cell damage- and oxidative stress-resistance • in humans physiological and pathological polyploid cells may coexist Megakaryocytes and polyploidy - formed in bone marrow by incomplete mitosis - highly polyploid structures - Practical implication: - 32-ploid megakaryocyte can form 3000 platelets, vs. - if identical number of divisions by mitosis, only 16 platelets Polyploidy formation by cell fusion • PC-3 tumor cells under increased stress (chemotherapy treatment) • evolution-driven pro-survival mechanism Senescence in pathological processes • Senescence initiation is beneficial/harmful, based on disease • Conditions where senescence is beneficial: • Counteracting tumor progression (when intensity of oncogenic signals reach threshold, p53 pathway activates) • Post-infarction cardiac fibrosis senescence improves • Liver fibrosis atenuation senescence restricts fibrosis. • Skin scarring restriction • Conditions where senescence is harmful • Conditions where senescence play a role: senescence is beneficial senescence is detrimental role, but unknown senescence generally beneficial in short-term stress condition. prolonged senescence activation associated with complications Diabetes mellitus Senescence as a therapeutic target Demonstating cause and effect in biology • to eliminate gene or proces and determine the phenotype Dual role of senescence • „knockout“ strategies does not work to prove senescence: • organisms which cannot undergo senescence do not live longer, • rather, they prematurely die of cancer j.gumulec@med.muni.cz | @jarogumulec | www.med.muni.cz/masariklab